The invention relates to a method for the hydrolysis of hydrolyzable solid metal salts with aqueous salt solutions.
In the conversion of chlorosilanes from metallurgical silicon and HCl gas to form trichlorosilane (TCS) and silicon tetrachloride (STC), which are used for example as starting materials for the production of semiconductor-pure silicon, solar silicon as well as highly dispersed silicic acid, metal salts, principally metal chlorides, are formed as byproducts. In particular aluminum chloride and iron chloride are precipitated as solids on cooling the reaction mixture. A targeted separation of the solids is important for process technology reasons, in order to avoid disruptions in the distillative working-up of the liquid silanes. Various methods are known for separating the solid metal chlorides from the process.
DE 2623290 A1 describes the precipitation of AlCl3, mixed with FeCl3, from the gaseous reaction mixture of chlorosilanes in Liebig tubes. The cleaning of the Liebig tubes is not described.
Since the solids are as a rule mixtures of highly corrosive compounds, the immediately following hydrolysis and possible neutralization of the resultant aqueous solutions is preferred to a purification of the solids (for a utilization). In the hydrolysis of the anhydrous metal chlorides (in particular of the main constituent aluminum chloride) it is known that a very large amount of heat is released, and if the heat is not sufficiently dissipated this can lead to evaporation of the water and as a consequence to an uncontrolled pressure build-up and thus to dangerous plant conditions and even an explosion.
For safety reasons apparatus and plant parts that are contaminated with these metal chlorides therefore have to be dismantled for cleaning by hydrolysis. This involves, apart from the danger of accidents when opening the plant and transporting the contaminated apparatus, a high logistical effort and expenditure in order to avoid downtimes. The associated safety risks as well as the costs involved are considerable.
As an alternative a method is described for example in EP 1174388 A1, in which the metal chlorides after precipitation in a quenching device are filtered off in a complicated manner and thereby isolated in a usable form. Since however as a rule mixtures of metal chlorides are involved, here too the hydrolysis and subsequent disposal via an effluent treatment plant is economically more advantageous than a further costly and complicated purification. The hydrolysis can then be controlled via the metering rate of the filter cake.
DE 4116925 C describes the recovery of the chemically bound chlorine in the distillation sump of the direct synthesis of chlorosilanes or organochlorosilanes, as hydrogen chloride by conversion with sulfuric acid. This however requires an additional complicated process technology plant for releasing and purifying the HCl and neutralizing the sulfuric acid residue.
The hydrolysis of AlCl3 in hydrochloric acid solution is described in “Heats of dilution of the hydrolyzing electrolytes AlCl3, Th(NO3)4, and UO2(NO3)2 at 25° C.”, Lange, E.; Miederer, W., Univ. Erlangen, Germany, Zeitschrift fuer Elektrochemie and Angewandte Physikalische Chemie (1957), 61 407-9. In this case, in contrast to hydrolysis in pure water, surprisingly even positive hydrolysis enthalpies were measured. The use of hydrochloric acid is however costly and also problematic, especially with regard to the disposal of the hydrolysis products via an effluent treatment plant on account of the additional water contamination and the high corrosiveness.
The removal of AlCl3 from liquid TiCl4 with the aid of NaCl/water at high temperature is described in U.S. Pat. No. 4,125,586 A.
The object therefore existed of hydrolyzing hydrolyzable metal salts in a simple and safe way.